The present invention relates to a trouble detecting apparatus for a sensor in an internal combustion engine system, and more particularly, to a trouble detecting apparatus for a sensor which generates a periodical detecting signal in relation to the engine speed of the internal combustion engine.
In a system for electronically controlling an internal combustion engine, a rotational speed sensor is employed as a signal generator which produces an electrical signal indicative of the engine speed, and this sensor is arranged to generate a pulse each time the crankshaft of the engine rotates by a predetermined angle. Since the interval between successive pulses becomes short at high engine speed and long at low engine speed, the engine speed can be detected from the interval between the pulses. Therefore, when no pulses are generated on account of some trouble in the rotational speed sensor, the condition becomes the same as when the engine speed is zero. In this case, since the control circuit cannot distinguish the difference between sensor failure and zero engine speed, such a trouble has a great effect on the control of the engine operation. When the system is provided with another sensor, such as a needle valve lift sensor, which generates an electric signal by which it is possible to discriminate whether or not the engine is rotating, any trouble in the sensor can be detected rather easily. For example, such a conventional apparatus for detecting troubles in the sensor is disclosed in U.S. Pat. No. 4,395,905. However, in cases where for the sake of simplicity only a rotational speed sensor is provided, the fact that the detected period of the output signal from the rotational speed sensor becomes more than a predetermined value is taken to indicate malfunction of the sensor. Consequently, it is impossible to distinguish between the stopped condition of the engine and malfunction of the sensor.